1. Field of the Invention
The present invention relates to a device having an element that is composed of a luminous material sandwiched between electrodes (hereinafter referred to as light emitting element) (the device will be called a light emitting device), and to a method of manufacturing the same. The invention particularly relates to a light emitting device using a light emitting element formed of a luminous material that provides EL (Electro Luminescence) (hereinafter the material and the element are referred to as EL material and EL element, respectively). This device will be called an EL light emitting device. Note that the light emitting device of the present invention includes an organic EL display and an organic light emitting diode (OLED).
EL materials usable in the present invention include every luminous material that emits light (phosphorescence and/or fluorescence) through one or both of singlet excitation and triplet excitation.
2. Description of the Related Art
An EL light emitting device has an EL element structured to have an EL material film (hereinafter referred to as EL film) sandwiched between an anode and a cathode. A voltage is applied between the anode and the cathode to cause a current to flow through the EL film, whereby carriers are recombined and the element emits light. In other words, an EL light emitting device is capable of emitting light by itself owing to the light emitting element included therein, and hence the device does not need any backlight unlike a liquid crystal display device. Furthermore, the EL light emitting device has advantages in that the viewing angle is wider and it weighs lighter.
The mainstream structure of an EL layer for an EL element is currently a laminate structure proposed by Tang, et al. with Eastman Kodak Company which has three to four layers. This structure is characterized in that the EL layer is divided Into an electric charge transportation layer and a light emitting layer by their functions. The term EL layer herein refers to both a single layer of EL film and a laminate in which the EL film and other organic films or inorganic films are layered.
Recently, however, concern has been expressed about an increase in manufacture cost due to complication of the manufacturing process which is brought by the multilayered EL element. In the wake of this concern, attempts have been made to reduce the number of layers that constitute the EL layer by doping a light emitting layer with a specific impurity element so that the light emitting layer can also serve as an electron transportation layer and an electron injection layer.
For instance, Idemitsu Kosan Co., Ltd. has proposed to dope a superficial region of a light emitting layer (distyryl arylene derivative) with cesium (Cs) or like other low work function elements by coevaporation so that the doped region can be used as an electron transportation layer (The 6th FPD Seminar Preliminaries, pp. 83-88, hosted by Journal of Computer Aided Chemistry).